I don't believe it's any of your employer's business what you do on your own time but unfortunately many employer's don't think that way. Like drugs and alcohol, working a second job means to them that you can't give them 100%. Some employers will even make you sign an agreement not to have any other job. It may also concern them that at some point you may quit to pursue your own business. If you receive benefits through your employer, often risky (as compared to the work you do for your employer) after work activities will raise eyebrows with the insurance carrier and could be a reason for them to increase what your employer pays. I could see that possibly happening with doing something like tree work.

So I believe the best policy is to keep your mouth shut about what you do with your own time.